Process of centrifugally separating minute porous particles



may.

United States Patent 3,185,301 PROCESS 0F QENTRIFUGALLY SEPARATHNG MINUTE POROUS PARTICLES Hillis 0. Folkins and Gral L. Beber, Crystal Lake, Ill., assignors to The Pure Oil Company, Chicago, 11]., a corporation of (lhio No Drawing. Filed Get. 26, 1960, Ser. No. 64,984

7 Qiaims. (Cl. 2&9-3)

The prior art teaches a method for the separation of micron-size particles into discrete fractions, in which the particulate material is fluidized in a high velocity gas stream and the stream is directed in a generally circular course. Centrifugal force causes the larger-size particles to migrate toward the surface of the confining vessel, While the finer particles remain fluidized and pass inwardly with the gas stream to an efiiuent duct which may be located centrally of the circular course. A portion of the finer particles which remain fluidized separates from the carrier gas as it enters the effluent duct, but some extremely fine particles remain fluidized and are exhausted through the duct. The separated fraction may pass downward, by gravity, into a receiving vessel.

Such apparatus for the separation of selected fractions of micron size particles is commercially available, but has been found in practice to be incapable of producing discrete fractions of very small size range, where the average particle size of the fraction is less than about ten microns. Where the average particle size of the desired fraction is about 3 microns or less, the problem of classification becomes acute.

It is an object of this invention to provide an improved method for classifying particulate material into a discrete fraction having an average particle size of less than about 10 microns. It is another object of this invention to provide an improved process for classifying particulate matter, by the use of commercially-available separation equipment, into narrower discrete fractions than have heretofore been possible. Yet another object of this invention is to provide an improved process by which particulate material may be classified to produce a discrete fraction having an average particle size of not greater than about 3 microns, in which the size range within the fraction is less than about 1 micron.

This invention is based in part upon the discovery that porous particulate material which has been wetted with a quantity of a liquid suflicient to substantially increase the density thereof may be conveniently classified into more narrow fractions than can the dry material. The method of this invention contemplates the use of conventional apparatus for the separation of fluidized particles. feed material without substantially altering the particle size thereof, or may be one which both attrites the particles to reduce their average size and classifies the attrited particles. For example, the micronizer type of device, which is well known and commonly used, both attrites and classifies the pulverized feed material. Such devices, which are also known as jet mills, are capable of reducing the average size of the feed material to as little as one micron or smaller. In these devices, a feed of appropriate mesh size is entrained in a ring manifold by a gas stream, which may be air, from which the feed drops through ports into a shallow circular grinding The apparatus may be one which classifies the q "ice spaced around the periphery of this chamber, the openings being at an angle between tangential and radial, so

- that a strong rotational motion is imparted to the fluid in the chamber as it moves inwardly. The larger particles are carried to the periphery of the chamber where they flow circularly in a suspension layer and pass close to each jet. The shock impact of the jet upon this layer causes the particles to strike each other and become attrited; When the particles become suficiently fine to pass inwardly with the flow of gas, against the centrifugal action tending to retain them adjacent the periphery of the chamber, they move inward to the gas outlet duct where most of them separate from the efiiuent gas stream.

Other classifiers merely utilize centrifugal separation action upon a fluidized stream, and thereby classify the particulate feed without substantially reducing the average particle size thereof. Cyclone-type classifiers are examples of devices in which the average particle size is not substantially altered. The method of this invention is equally applicable to either the micronizer or cyclonetype of systems.

In accordance with this invention, the efliciency of separation is greatly improved by partially saturating porous solids of the size range to be separated with an aqueous or organic liquid to an extent sufiicient to at least partially fill the particle pores with the liquid, but insuflicient to adversely affect the fluidizable condition of the particles. The partially-saturated porous solid, for example, may be silica gel or alumina gel. Then, the liquid-containing particles are separated in a conventional manner by mean of centrifugal classifiers. The separated fraction may then be dried or calcined to its original density.

In many cases, where the separation of porous solids, or the production. of porous solids, in a given range of particle diameters is desired, it is possible to start with small particles of relatively larger diameter, say 10 to 30 microns, and treat them in the manner described above. The larger-than-desired particles which have been treated with the liquid are then processed in a micronizer-type device to reduce the particle size and to separate the resulting attrited particles which fall in the range of the selected fraction.

An advantage of this invention is that the method controls or prevents the build-up of high charges of static electricity upon the particles during separation, thus enhancing separation efficiency.

It is preferred that the porous material to be classified not be exposed to high drying or calcination temperature. Thus, if the porous solid is a silica gel, it is preferred that the calcination of the precursor hydrogel be not too severe, i.e., that thecalcination temperature not exceed 800 F., and preferably, that the calcination temperature not exceed 600 F. Silica gels which meet these requirements are commercially available. A preferred method is to dry or calcine the precursor hydrogel in a spray dryer at a temperature and for a time at which the resulting gel may be fluid'med by conventional methods. Depending upon its nature, the gel may contain from about 26 to about 45 percent by weight of water at this time. Care should be taken not to calcine the hydrogel too severely and thus finally set its structure. Hydrogels dried by this procedure may be separated into the desired minute particle size Without the re-addition of water.

If drying or calcining is carried beyond the point at which the porous material first ibecomes readily fluidizable, water or other liquids or solutions may be added back to the powdered porous material until the pores are filled. Since excess liquid will probably be added, the material is then dried until fluidizable properties are reestablished and the material, is then separated, or reduced Where the porous solid has been calcined at temperatures above 800 F., or to a pointwhere the structure of the gel is firmly set, it is preferred to add, as a component of the liquid, a small amount of an organic or mineral acid to partially re-peptize the porous solid particle. Suitable acids, for example, are acetic acid and dilute solutions of hydrochloric or sulfuric acid.

Following the separation of the particulate material into its small micron-range classification, the separate fractions maybe dried or further calcined, preferably in a spray drier or cmried in inert gaseous medium, to the desired water level at which the porous solid is to be finally used, e.g., such as finely divided catalsyts or fillers or additives for various chemical compositions. Since, according to the method of this invention, the liquidcontaining porous solids have not shrunk to their ultimate size when they are charged to the separator, the product particles will be somewhat smaller after being heated to drive off the water or other liquid than when they were discharged from the mechanical separators. Thus, this is anotheradvantage of the invention in that it permits separation of unfinished particles at particle diameters larger than thoseof the desired finished particles. In general, the particles will be about -100% larger during the separation step than after they have been calcined at temperatures up to about 1000" F.

Although aqueous solutions are quite suitable for use as the pore-filling liquid, it is sometimes preferable to add liquids or solutions of somewhat higher boiling point than water in order to minimize evaporation of theliquid during the separation process. Alcohols, glycols, ethers, ketones, or other organic compounds, especially those having a high solubility in water, are preferred. Where the absorbed liquid is water, it is preferred that the fluidizing gas in the classification process be maintained under conditions such as to minimize the evaporation of water during the classification. This maybe done by maintaining the water vapor content of the gas at a levelpreferably not less than about 80% of saturation. The fluidizing medium is generally air or steam. If air is employed, relative humidities in the rangeof 80 to 95% will be found satisfactory. Where the fiuidizing medium is steam, it is preferred that the steam not' be superheated beyond about of superheat. Preferably, the steam will be substantially saturated. This is especially so where the conditions under which the steam is expanded are such that the gas upon expansion will become superheated. i

The particulate material may be Wetted by'shnply adding a suitable quantity of liquid to the solid particles and subjecting the mixture to mechanical agitation. In order to substantially fill the particle pores Without impairing the fluidizability of the particles, it'is preferred to mechanically mix the particulate matter with a quantity of liquid about double that required to saturate the pores of the particles. After agitation, the excess liquid is rem ved by drying the material either at ambient or moderately elevatedtemperatures, with or without the application of a vacuum.

As a specific example of the method of this invention, a sample of silica gel microspheres about microns in diameter is ground and separated into particle sizes ranging from about 10 microns to about 0.5 micron in diameter. The original sample is wetted With water in an amount equivalent to 0.7 part by weight of water per part of original silica gel. The wetted mass is then dried at ambient temperature until the material is in a fiuidizable state, at which time it contains about 0.35 part of water per part of original silica gel. The thus-conditioned silica gel is then classified by means of a frnicronizer-type separator into a discrete fraction having an average particle diameter of 2 microns, the particles comprising said fraction having a diameter range of 1 micron. The carrier fluid employed is air, maintaincd at a relative humidity of The selected silica gel fraction is then calcined at moderate temperatures untilits initial density isle-established. V

Although the invention is drawn primarily to the separation or grinding and separation of porous solids into particles,.ranging from about 0.5 or less to about 10 microns, the method may be employed for producing high temperature ceramic-like materials, such as silica or alphaalumina',' as 'fine powders made up of particles ranging from less than 0.5 micron to 10 microns in diameter. This modification involves only one subsequent additional step. The finely divided porous material (about 0.5 to 10 microns) is carried in inert gas through a high temperature furnace at about 2000-3000 F. to sinter the finely divided porous particles, thus producing ceramic or non-porous material of higher density and low surface alumina, to produce a narrow fraction having an average particle size in the range of about 0.5 to 10 microns by entraining unclassified particles in a high velocity "gas stream and directing said stream in a generally circular course, the improvement comprising treating said particles With at least about 0.7 part by weight of water per part of particles, drying the resulting particles to a fluidizable state containing about 0.35 part of added water per part 'of initial material, said last mentioned amount of added water being sufiicient to at least partially fill the particle pores before entraining said particles in said gas stream, Without causing said particles to be non-fiuidizable in said gas stream, and maintaining said gas stream under conditions such as to minimize the evaporation of said water during the classification of said particles.

2. A method in accordance with claim 1 including the final'step of heat drying said material to substantially the initial Water content thereof.

3. A process in accordance with claim 2 in which the size difference of particles comprising said fraction does not exceed about 1 micron.

4. A process in accordance with claim 3 in which the average particle size o f said fraction is in the range of 0.5 to 3 microns.

'5. In the preparation of materials'of the group consisting of silica gel and alumina gel from precursor hydrogels by calcination and centrifugal classification to produce a narrow particle fraction having an average particle size in the range of about 0.5 to 10 microns by entraining a mixture of particles of varying size in a high velocity gas stream and directing said stream in a generally circular course, the improvement comprising first calcining said precursor hydrogels to a Water content of about 20% to about 45% by weight, then classifying the particles, while maintaining said gas stream under conditions such as to minimize the evaporation of said liquid during the classification of said particles, and calcining the resulting narrow fraction to the'desired water content.

6. A method in accordance with claim 5 in which the size difference of the particles comprising said fraction does not exceed about 1 micron.

7. The process in accordance with claim 6 in which 5 6 the average particle size of said fraction does not exceed 2,832,470 4/58 Rietema 209-1725 about 3 microns. 2,856,268 10/58 Young 23-182 References Cited by the Examiner HARRY B. THORNTON, Primary Examiner.

UNITED STATES PATENTS 5 ROBERT OLEARY, EDWARD J. MICHAEL, HER- 2 433 372 9 49 Payne 2()9 172 BERT L. MARTIN, GEQRGE D. MITCHELI 2,508,867 5/50 Rampino 209-4 Examiners- 2,642,185 6/53 Fontein 209-211 

5. IN THE PREPARATION OF MATERIALS THE GROUP CONSISTING OF SILICA GEL AND ALUMINA GEL FORM PRECURSOR HYDROGELS BY CALCINATION AND CENTRIFUGAL CLASSIFICATION TO PRODUCE A NARROW PARTICLE FRACTION HAVING AN AVERAGE PARTICLE SIZE IN THE RANGE OF ABOUT 0.5 TO 10 MICRONS BY ENTRAINING A MIXTURE OF PARTICLES OF VARYING SIZE IN A HIGH VELOCITY GAS STREAM AND DIRECTING SAID STREAM IN A GENERALLY CIRCULAR COURSE, THE IMPROVEMENT COMPRISING FIRST CALCINING SAID PRECURSOR HYDROGELS TO A WATER CONTENT OF ABOUT 20% TO ABOUT 45% BY WEIGHT, THEN CLASSIFYING THE PARTICLES, WHILE MAINTAINING SAID GAS STREAM UNDER CONDITIONS SUCH AS TO MINIMIZE THE EVAPORATION OF SAID LIQUID DURING THE CLASSIFICATION OF SAID PARTICLES, AND CALCINING THE RESULTING NARROW FRACTION TO THE DESIRED WATER CONTENT. 